1. Field of the Invention
The present invention relates to a process for the preparation of 2,6-pyridinedicarboxylic acid esters by reaction of halogenated pyridines with carbon monoxide and an alcohol in the presence of a base and a catalyst. 2,6-Pyridinedicarboxylic acid esters are important intermediates, e.g., for the preparation of compounds having anti-inflammatory action (Japanese Published Patent Application (Kokai) No. 93/143799). The 2,6-pyridinedicarboxylic acid esters which can be prepared according to the invention have the general formula: ##STR2## wherein R.sup.1 is a C.sub.12 -C.sub.6 -alkyl group, a C.sub.3 -C.sub.6 -cycloalkyl group, an aryl group or an arylalkyl group, and R.sup.2 and R.sup.3 independently of one another are hydrogen or chlorine and R.sup.4 is hydrogen, a C.sub.1 -C.sub.6 -alkyl group, a C.sub.1 -C.sub.6 -alkoxy group or fluorine.
2. Background Art
Known processes for the preparation of 2,6-pyridinedicarboxylic acid esters are based on the direct chemical oxidation of 2,6-dimethylpyridine (I. Iovel, M. Shymanska, Synth. Commun., (1992), 22, 2691; G. Wang, D. E. Bergstrom, Synlett (1992), 422; Belgian Patent No. 872,394; Soviet Patent No. 568,642). Although high yields are in some cases achieved here, the processes are associated with disadvantages because of the use of expensive and/or toxic reagents. A process for the preparation of 2,6-pyridinedicarboxylic acid esters by carbonylation of 2,6-dichloropyridine using a nickel catalyst is known [International Published Patent Application No. (WO) 93/18005]. A disadvantage of this process is that the reaction is carried out at high pressure, the 2,6-pyridinedicarboxylic acid esters are only obtained in moderate yields and a high proportion of monocarbonylated side product is formed.